為了進一步改善LLC型半橋諧振式轉換器的維持時間(Hold-up time)，本論文提出一種電流控制激磁電感值之變壓器，當前級電路輸入電壓關斷時提供所需之激磁電感值；具體而言，在輸入電壓關斷而由輸入電容提供所需能量情況下，輸入電壓將不斷下降，使LLC型半橋諧振式轉換器所需之最小激磁電感同時降低，因此利用步階式氣隙變壓器用以調變激磁電感來增加電壓增益，能夠延長半橋串聯諧振式轉換器之維持時間，且無需使用其他輔助及控制電路。
本論文提出步階式氣隙變壓器之感值與電流關係的數學模型，並利用一台輸入電壓380V、輸出電壓19V，輸出150W之LLC型半橋諧振式轉換器架構實際測試並驗證，步階式氣隙變壓器用於LLC型半橋諧振式轉換器於同樣輸入電容情況下能夠有效提升維持時間。

A current-controlled variable transformer’s magnetizing inductance strategy, for further improving the hold-up time on half-bridge LLC converter, is proposed. A simple control strategy is used to set the desired inductance value when the input power is cut-off, In detail, the transformer’s magnetizing inductance decrease sharply when the input power is cut-off, therefore, the converter’s hold-up time can be improved without using additional auxiliary circuit or modulation control method. The mathematic model between magnetizing inductance and step-gap of LLC transformer is proposed, the benefit effects using this control strategy are verified theoretically. In the end, two platforms of LLC resonant converter are built. The experimental results have verified the hold-up time improvement of LLC resonant converter utilizing step-gap transformer.

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